The present invention generally relates to bromination processes. More particularly, the invention relates to processes for brominating substrates such as non-fused aromatic compounds, to form brominated materials which are useful as flame retardants.
Brominated compositions are commonly compounded into flammable materials such as polymers to serve as flame retardants. Such compounding operations are heavily dependent upon the physical properties of the flame retardant, and thus flame retardants with improper physical properties can be more difficult to effectively compound and confer poor properties to the resulting flame retarded formulation.
Brominated compositions which contain a mixture of brominated homologs and melt over broad temperature ranges are advantageous in compounding operations. Previously, mixed, brominated compositions of diphenyloxide, or DPO, have been prepared in batch reactions. In such reactions, solvents have been used, and the level of bromination has been controlled by controlling the amount of bromine added to the reaction mixture. In the case of brominations without solvent, DPO has been an effective substrate since it and its brominated analogs melt at temperatures sufficiently low to maintain a stirrable reaction mass while achieving the desired level of bromination. However, other non-fused aromatic compounds, for example diphenylalkanes, and their brominated analogs, melt at substantially higher temperatures. Consequently, brominations are complicated because the reaction mixtures must be driven to relatively high temperatures to maintain a workable mixture. Such high temperatures are generally deleterious to the formed product and lead to high color. As a result, batch brominations employing solvents have generally been proposed, e.g. in U.S. Pat. Nos. 5,055,235 and 5,041,687.
In light of the above-noted difficulties, a need exists for bromination processes which can be used to conveniently and reliably achieve products having desired levels of bromination for a broad range of non-fused aromatic substrates. Desirably, the bromination processes would provide mixed, brominated compositions which differ substantially in product distribution and have broader melting ranges than products of corresponding batch bromination processes. The present invention is addressed to these needs.